Transient voltage suppressor

ABSTRACT

A circuit for eliminating the effects of undesirable transient voltages generated by the removal of a primary source voltage from, or interruption of current flow through the windings of an inductive element.

United States Patent Albert W. Compoiy Marlboro;

Peter A. Vincent, Little Silver, both of, NJ. 81 1,927

Apr. l, 1969 Aug. 3, 197 l The Bendix Corporation Inventors Appl. No.Filed Patented Assignee TRANSIENT VOLTAGE SUPPRESSOR ll Claims, l0Drawing Figs.

U.S. CL 307/246, 307/254, 307/237, 328/67, 307/318 Int. CL H03k17/08,H03k 17/64 Field of Search 328/67;

[56] References Cited UNITED STATES PATENTS 2,294,388 9/1942 Dawson.....328/67 2,677,053 4/1954 Nims 328/67 2,909,659 10/1959 Woo 307/2682,916,640 12/1959 Pearson 328/67 3,337,748 8/1967 Rusch et al. 307/246Primary Examiner-Donald D. Forrer Assistant Examiner-Harold A. DixonAltorneys- Plante, Hartz, Smith & Thompson and James M.

Nickels ABSTRACT: A circuit for eliminating the effects of undesirabletransient voltages generated by the removal of a primary source voltagefrom, or interruption of current flow through the windings of aninductive element.

TRANSIIENT VOLTAGE SUPPRESSOR BACKGROUND OF THE INVENTION l. Field ofthe Invention The invention relates to the field of pulse-formingcircuits and more particularly to pulse-forming circuits utilizingtransformer coupling such as in static inverters.

2. Description of the Prior Art The use of transformer coupling isdesirable whenever electrical isolation or impedance matching maycontribute to the performance of the circuit. Transformers, however,possess properties of inductance and respond to interruptions of currentflow through their windings in accordance with Lenzs Law, e=L di/dt, theessential properties of which` cause current to tend to continually flowunidirectionally through the windings.

Thus, the instantaneous interruption of current in an inductive elementinduces an electromotive force within the inductive element and is ofsuch polarity as to oppose a change of current ilow in the externalcircuit. The inductive element may now be considered as a voltagesource. A prime problem that develops in such arrangements is that thevoltage source, effectively, is looking into a high-impedancecircuit-with the resulting effect of an extremely high transient voltagebeing developed at the terminals of the inductive element.

The resulting transitory voltage may have deleterious effects on thereliability perfomance of the circuit in several ways:

l. Standoff voltage levels of circuit components may be exceeded,thereby causing punch-through or breakdown in the dielectric propertiesof the components.

2. Insulation between the wires and/or windings of the inductive elementmay deteriorate or breakdown.

3. False signals may couple to adjacent wires and/or circuits.

4. Spurious electromagnetic signals may radiate causing electromagneticinterference in susceptible equipment.

A second means by which an inductive element may generate a transientvoltage is through the properties of Faradays Law, e--db/dt. When a stepvoltage is impressed in the primary winding of a transformer, the fluxgenerated in the core increases linearly with the time duration of thestep function. Upon removal of the step function voltage, the flux willdecay to the remnance or residual flux level. This decay of fluxgenerates a transitory voltage, d I /dt=-e, in the windings of thetransformer. This transient voltage may have the same deleteriouseffects on the reliability of the circuit as those heretofore described.

The present invention provides a means of eliminating the deleteriouseffects of the undesirable transient voltages generated by the removalof a primary source voltage from, or the interruption of a current flowthrough the windings of an inductive element, or interruption of currentflow in the windings such as may be affected by a step function pulse ofcontrollable duration.

SUMMARY OF THE INVENTION The present invention provides a circuit inwhich a low-impedance path is provided across the primary winding of atransformer for clipping the transient voltages generated at the instantof switch off of voltage or interruption of current flow to thetransformer.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a simplified schematicdiagram illustrating the prior art.

FIG. 2 is a simplified schematic diagram illustrating the invention.

FIG. 3 is a similar to the circuit of FIG. 2 with transistors as theswitching means.

FIG. 4 is a schematic diagram of a typical circuit embodying theinvention.

FIGS, 5a, 5b are curves representing the prior art. FIGS. 6a and 6b arecurves representing the present invention.

g FIGS. 7a and 7b are curves showing the inductive effect of iron cores.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. l of thedrawing, a push-pull circuit is indicated generally by the numeral 1 0and includes a transformer ll. The transformer 11 has a primary windingl2 having a center tap 13. One side of the winding I2 is connected by aconductor 14 to a stationary contact 15 of a switch member 16. The otherside ofthe winding 12 is connected by conductor 17 to a stationarycontact 18 of a switch member 19. The switch members 16 and 19 areconnected together by conductor 20. The center tap 13 of the winding 12is connected by conductor 21 to one side of a suitable source ofcurrent, indicated as a battery 22. The other side of the battery 22 isconnected by conductor 23 tothe conductor 20.

In addition to the primary winding l2, the transformer 11 has secondarywindings 24 and 25 and an iron core 26. The windings 24 and 25 areconnected by conductors 27, 27a, 28 and 28a respectively to a suitableload 29.

ln the operation, when switch 16 is closed, the circuit in one-half ofthe push-pull arrangement is completed and primary source voltage isimpressed across one-half of the primary winding 12 of the transformer11. Magnetizing current then flows through the primary winding l2 of thetransformer 1l generating an m.m.f. which drives the flux in the core.If after a time switch 16 is opened, with switch 119 remaining open, theprimary source voltage will be instantaneously removed from the primaryIwinding l2 and the magnetizing current will be abruptly interruptedcausing a transient voltage to be developed at the terminals of thewindings of the transformer 1,1. The second half of the cycle operatesin the same manner with switch 19 closed and switch 16 open. Thevoltages as described are illustrated in FIG. Sa for narrow pulses andFIG. 5b for wide pulses. The cycle is repetitive as shownin FIGS. 5a and5b. The switches 16 and 19 represent any switching components,mechanical, solid'state or other.

Referring now to FIG. 2 which is a modification of FIG. l and only thechanges will be described in detail. A resistor 30 has one sideconnected to the conductor 14 and the other side connected to stationarycontact 3l of a switch 32 which is connnected to the conductor 21. Aresistor 33 has one side connected to stationary contact 34 of a switch35 which is connected to the conductor 17.

ln the operation, the switches 32 and 35 are normally open. However, ifeither or both switches 32 and 35 are caused to close with thesimultaneous opening of either switch 16 or 19, a complete path forcurrent flow is provided through the switch 32, resistor 30 and theprimary windings l2 of the transformer l1, The energy released in thecircuit by the generation of a transient voltage wheneither switch 16 or19 are opened willbe directed to and dissipated in either or bothresistors 30 and 33. This results in the clipping of the undesirablevoltages present in FIGS. `5a and 5b. It is understood that theresistors 30 and 33 are of a relatively low ohmic value. The results ofthe clipping action is a relatively clean voltage pulse of controlledduration as shown in FIG. 6a for a narrow pulse and in FIG. 6b for awide pulse.

FIG. 3 depicts the circuit arrangement of FIG. 2 but with the switches16, 19, 32, and 35 replaced by transistors Q1, Q2, Q3 and Q4respectively. Also the internal resistance of transistors Q3 and Q4 maybe substituted for the resistors 30 and 33. The operation is similar tothat of FIG. 2. Either or both Q3 and Q4 are caused to conductsimultaneous with the switching of either transistor Q1 or Q2 to anonconducting State.

Referring now toFlG. 4 in which a typical circuit embodying theinvention is illustrated. A transformer 1 1 has a primary winding 12which has a center tap 13. One end of the winding 12 is connected byconductor 14 to collector 36 of transistor 37. The transistor 37 has abase 38 connected to one output of I a switching circuit 39 and anemitter 4t) connected by conductor 4l to emitter 42 of a transistor 43.The transistor 43 has a base 44 connected to another output of theswitching circuit 39 and a collector 45 connected by conductor R7 to theother side of the winding l2.

The center tap t3 of the winding lll is connected by conductor 13a tocollector 46 of a transistor 47. The transistor 47 has a base 48connected by conductor 49, resistor 50 and conductor 5l to collector 52of a transistor 53. The transistor 47 also has an emitter 54 connectedto a conductor 55 which is connected through diodes 56 and 57 betweenconductors 14 and 17. A diode 58 is connected between the emitter S4 andcollector 46. The base 48 of the transistor 47 is also connected byresistor 59 to the conductor SS.

The transistor 53 has an emitter 60 connected by conductor 6l and Zenerdiode 62 to the conductor 4l which is also connected to the negativeoutput of a suitable power source (not shown). The emitter 60 is alsoconnected by resistor 63 and conductor 64 to the positive terminal ofthe power source which is also connected by conductor 65 to the tap E3ofthe winding 1l. The transistor 53 has a base 66 connected by conductor68 to conductor 69 which is connected through diodes 70 and 7l betweenthe conductors 14 and B7.

In addition to the primary winding l2, the transformer lll has an ironcore 26 and secondary windings 24 and 25. The winding 24 is connected byconductors 27 and 27a to a suitable load 29. In like manner, the winding25 is connected by conductors 28 and 28a to the load 29.

The transistors 37 and 43 are overdriven amplifiers operating in theswitching mode so that alternately they are either full-on driven intosaturation, or "full-off," passing only their leakage current. Theswitching circuit may be any switching control that provides a means ofalternately driving the transistors 37 and 43 "on" and ofl`," andprovides means for 'controlling the conduction angles or, the durationsof the on" and off periods ofthe transistors. An example would be theforward stages of a static inverter.

When the transistor 37 is on," the transistor 43 is of and the currentpath from the positive terminal of the power supply to the center tap 13of the winding l2 of the transformer ll, through the winding 12 to thecollector 36 of the transistor 37, and through the transistor 37 to thenegative terminal of the power supply. This impresses the primary sourcevoltage across one-half` of the winding l2 of the transformer 1l. Duringthis time period, transistor 53 is biased to cut off by being derived ofbase drive current. Current flows from the positive terminal throughresistor 67, blocking diode 70 and switched-on" transistor 37 to thenegative terminal. The Zener diode 62 is kept alive" by bias currentflow from the positive terminal through the resistor 63, and Zener diode62 to the negative terminal and provides means for backwardly biasingthe transistor 53 when either transistor 37 or 43 is conducting. Whenthe transistor 53 is in cutoff, the transistor 47 is also in cutoff asthe flow of base drive current is interrupted in its flow throughresistor 50 and cutoff transistor 53.

When the transistor 37 is switched off and the transistor 43 has not yetbeen driven on, a dwell time" exists in which the primary source voltageis not impressed on either half of the primary winding l2 of thetransformer lll. During the dwell time, transient voltages are generatedin the windings of the transformer ll in accordance with Lenz's Lawwhich tends to keep the current flowing unidirectionally in accordancewith Faradays Law for changing magnetic fields. During the dwell time,the transistor 53 is driven on by biasing current flowing from thepositive terminal, through the resistor 67, baseemitter junction of thetransistor S3 and through the Zener diode 62 to the negative terminal.The transistor 47 also is driven on during the dwell time by biascurrent flowing from the positive terminal, diode 58, base-emitterjunction of the transistor 47, resistor 50, transistor 53, and throughZener diode 62 to the negative terminal, With transistor 47 on" alow-impedance path is provided through diode 56 across onehalf ofprimary winding l2 enabling the energy in transformer lll to bedissipated without generating high-transient voltages. Diode 57 providesthe low-impedance path with transistor 47 for the dwell time aftertransistor 43 has stopped conducting. Thus, during the dwell time and inresponse to transistor 37 or 43 being switched to the off-state arelatively low-impedance path is presented across the primary winding l2of the transfomier l1. This provides means for clipping the transientvoltages generated at the instant of switch off of the transistors 37and 43.

When transistor 47 is off, the resistor 59 provides a lowimpedance pathwhich is in shunt with the base-emitter junction of the transistor 47,providing means for circuit leakage currents to flow without biasing onthe transistor 47.

In like manner, when the transistor 43 is switched on, the second halfof the cycle occurs. The transistors 53 and 47 are off until thetransistor 43 is switched off, at which time they are once againswitched on to present a low-impedance path to the primary winding l2 ofthe transformer lll,

The use of iron alloy cores in inductive elements introduces furthercomplications which results in the generation of undesirable transientvoltages. Iron alloy cores, such as silicon steel, nickel-iron alloysand more important those cores producing square hysteresis loopcharacteristics are highly sensitive to the magnetization effectsproduced by an m.m.f. The nature of the iron core element is to exhibita balanced flux hysteresis loop which is symmetrically displayed aroundthe intercepts of the X-, Y-axes for each full cycle of excitation whenthe driving magnetomotive force is symmetrically applied over each fullcycle to the core, see FIG. 7A. During these conditions A B, thedifference between maximum flux density and residual flux density, maybe very small and the resulting transient voltage produced when theexcitation force is removed, and the flux density changes from Bmax toits residual level, may also be small. However, under actual operatingcondition, because of slight variations in commercially availablecircuit elements, such as transistors, diodes, resistors, etc., and incore materials themselves, the driving magnetomotive force may not besymmetrical about the Y-axis which results in the generation of anasymmetrical flux hysteresis loop, see FIG. 7B. The results of thisasymmetry is equivalent to the presence of a direct current component inthe rn.m.f., and causes the core to tend to saturate on one-half of thecycle as illustrated in FIG. 7B. This asymmetry, therefore, has theeffect of increasing A B and thereby generating relatively largeundesirable transient voltages. The low-impedance path as heretoforedescribed provides means for the direct current component to circulate,thereby resetting the flux hysteresis loop symmetrically about the X-,Y-axes and reducing the energy level of the transient voltages normallyproduced.

Although only a few embodiments of the invention have been illustratedand described, various changes in the form and relative arrangement ofthe parts, which will now appear to those skilled in the art, may bemade without departing from the scope of the invention.

What we claim is:

l. Circuit means for suppressing transient voltages comprising incombination, switching mans, a transformer having primary and secondarywindings, circuit means connecting said switching means to a powersource and to said primary winding of said transformer for energizingsaid transformer when said switching means is in the on-state, and meansresponsive to said switching means being switched to the off-state toprovide a low-impedance path across said primary winding for clippingtransient voltages generated at switch off of current flow to saidtransformer.

2. The combination as set forth in claim l in which said switching meansform a push-pull circuit.

3. The combination as set forth in claim l in which said switching meansare transistors.

4. The combination as set forth in claim l in which said transformer hasan iron core.

5. The combination as set forth in claim l inwhich said means forproviding a low-impedance path across said primary winding includes atransistor biased in a nonconducting state when current is flowing tosaid transformer and biased in a conducting state upon the interruptionof current flow to said transformer.

6. The combination as set forth in claim 5 having a second transistorbiased to a nonconducting state by a Zener diode and arranged to biasthe first-mentioned transistor in a nonconducting state when current isflowing to said transformer.

7. The combination as set forth in claim l in which said lastmentionedmeans includes a transistor and a diode to provide a low-impedance path.

8. The combination as set forth in claim 7 and including means forpreventing leakage current from biasing said lastnamed transistor to aconducting state.

9.1The combination as described in claim 1 in which the switching meansincludes a pair of transistors connected in a push-pull circuit acrosstwo halves of the primary winding of the transformer to alternatelyenergize the two halves of the primary winding.

l0. The combination as described in claim 9 in which the means toprovide a low-impedance path across the primary winding includes a pairof diodes connected in series with the primary winding and a transistorconnected to a junction between the diodes and to a center tap of theprimary winding to provide a low-impedance path across each half of theprmary winding of the transformer.

l1. The combination as described in claim l0 including means forrendering the lastmentioned transistor conducting when the switchingmeans is nonconducting.

1. Circuit means for suppressing transient voltages comprising incombination, switching mans, a transformer having primary and secondarywindings, circuit means connecting said switching means to a powersource and to said primary winding of said transformer for energizingsaid transformer when said switching means is in the on-state, and meansresponsive to said switching means being switched to the off-state toprovide a low-impedance path across said primary winding for clippingtransient voltages generated at switch off of current flow to saidtransformer.
 2. The combination as set forth in claim 1 in which saidswitching means form a push-pull circuit.
 3. The combination as setforth in claim 1 in which said switching means are transistors.
 4. Thecombination as set forth in claim 1 in which said transformer has aniron core.
 5. The combination as set forth in claim 1 in which saidmeans for providing a low-impedance path across said primary windingincludes a transistor biased in a nonconducting state when current isflowing to said transformer and biased in a conducting state upon theinterruption of current flow to said transformer.
 6. The combination asset forth in claim 5 having a second transistor biased to anonconducting state by a Zener diode and arranged to bias thefirst-mentioned transistor in a nonconducting state when current isflowing to said transformer.
 7. The combination as set forth in claim 1in which said last-mentioned means includes a transistor and a diode toprovide a low-impedance path.
 8. The combination as set forth in claim 7and including means for preventing leakage current from biasing saidlast-named transistor to a conducting state.
 9. The combination asdescribed in claim 1 in which the switching means includes a pair oftransistors connected in a push-pull circuit across two halves of theprimary winding of the transformer to alternately energize the twohalves of the primary winding.
 10. The combination as described in claim9 in which the means to provide a low-impedance path across the primarywinding includes a pair of diodes connected in series with the primarywinding and a transistor connected to a junction between the diodes andto a center tap of the primary winding to provide a low-impedance pathacross each half of the primary winding of the transformer.
 11. Thecombination as described in claim 10 including means for rendering thelast-mentioned transistor conducting when the switching means isnonconducting.